1. Field of the Invention
The present invention relates to an apparatus for simultaneously applying a plurality of layers onto a moving support or web. Particularly, the present invention relates to an improvement in an extruder used in an apparatus for simultaneously applying a plurality of layers of magnetic recording media. The extruder is of the type provided with doctor edges formed at a top end part of the apparatus, whereby coating compositions continuously extruded toward the surface of a moving support are applied to a uniform thickness onto the surface of the support through the doctor edges.
The term "support" used in this specification includes flexible sheets or webs formed of resin film, paper, polyolefin-coated paper, metal e.g., aluminum, or the like. Further, the supports of this type may have a sub layer or the like. The supports of this type are coated with various kinds of coating compositions, such as magnetic coating compositions, photographic light-sensitive coating compositions, or the like, to produce magnetic tape, various kinds of photographic film and paper, or the like.
2. Background of the Invention
As magnetic recording media have been developed into high density and thin film form in recent years, the structure of magnetic layers has been shifted from a monolayer structure to a multilayer one.
Supports provided with sub layers or the like have an advantage in that the supports adheres firmly to magnetic layers or back layers. Further, media having multiple magnetic recording layers have an advantage in that the capacity of magnetic data storage is remarkably improved compared to that in media having a single magnetic recording layer. Accordingly, structures of multiple layers, such as, for example, two layers or four layers, have been required in the recent years.
On the other hand, heretofore such a multilayer structure has been attained by repeating the coating and drying process layer by layer.
The method, however, suffers from poor production efficiently, because the coating and drying must be repeated. Further, it has a disadvantage in that undesirable modulation signals often occur due to an evenness of magnetic recording elements in the interface between coating layers.
Hence, a method of simultaneously applying and drying multiple layers by a single coating and drying process would be desired. However, it is self-evident that some conventional coating methods such as a roll coating method, a gravure coating method and a combination method of doctor coating and roll or extrusion coating, which have been mainly used for manufacturing magnetic recording media, are unsuitable for simultaneous formation of multiple layers.
On the other hand, a slide coating method employing slots has been proposed as one of multilayer coating methods for manufacturing photographic light-sensitive materials. Further, Japanese Patent Publication No. 56-12937 (1981) has disclosed a multilayer coating method for magnetic recording materials in which a coating composition to be applied is caused to flow out along a slide surface of a slide hopper. Such slide coating methods, however, are unsuitable for manufacturing magnetic recording media, because the coating composition to be used is a highly viscous and aggregative "organic solvent dispersed liquid", which is easily dried and unsuited to high-speed application.
Further, various extrusion type coating methods have been proposed but most of the methods are merely suited to monolayer coating.
On the other hand, Japanese Patent Application Laid-open No. 58-109162 (1983) has disclosed a multilayer forming method using an extrusion coating head to be pressed to a support. However, the structure of the coating head described in the Japanese Patent Application Laid-open No. 58-109162 (1983) is not sufficient to solve the aforementioned problems. The method still has many problems, in which coating trouble, such as streak-like irregularity, thickness irregularity and the like, often occurs when multiple magnetic recording layers are formed. Further, for example in Japanese Patent Application Laid-open No. 60-238179 (1985), a coating head using an R-shaped doctor edge has been disclosed. It is known that the aforementioned problems in streak-like irregularity and the like can be eliminated to some extent by the R-shape structure. However, that shown in the Japanese Patent Application Laid-open No. 60-238179 (1985) is not such a multilayer coating method as is the subject of the present invention but only a monolayer coating method. Generally a coating mechanism in the simultaneous multilayer coating method is relatively complex compared to that in the monolayer coating method. Accordingly, no means has been disclosed which is effective for the simultaneous multilayer coating method.
Further, as a method using an extrusion type coating head, a multilayer coating method has been disclosed in Japanese Patent Publication No. 49-29944 (1974) and Japanese Patent Application Laid-open No. 50-138036 (1975). According to the Japanese Patent Publication No. 49-29944 (1974), a doctor edge surface is shaped like a plane. For this reason, the aforementioned problem in streak-like irregularity and thickness irregularity still has arisen so that the thus produced products have been scarcely fit for use with respect to their surface quality. According to the Japanese Patent Application Laid-open No. 50-138036 (1975), the doctor edge is further improved to have a curved surface. However, it fails completely to disclose conditions for simultaneous multilayer coating at all.
In short, there has been disclosed no effective means to clear up how to apply the first layer and how to apply the second layer without a deleterious influence on the first layer.